In recent years, high performances, miniaturization and portability of electronic equipments are far advanced thanks to progress in electronic technologies. Consequently, researches on a rechargeable secondary battery are directed at providing a power supply which could be used conveniently and economically for a long time. Conventionally, as a secondary battery, a lead storage battery, an alkali storage battery, and a secondary lithium-ion battery are known widely. Especially, the secondary lithium-ion battery attracts attention as a battery which can realize high output and a high-energy density.
In the secondary lithium-ion battery, for example, a metal oxide, metal sulfide, or polymer is used as a positive electrode material. Specifically, non-lithium compounds such as TiS2, MoS2, NbSe2, or V2O5, or lithium containing oxides such as LiCoO2, LiNiO2, LiMnO2 or LiMn2O4 are known.
Among these materials, LiCoO2, which has been widely put in practical use as the positive electrode material having about 4 V potential to a lithium metal potential, has a high-energy density and a high voltage, and is an ideal positive electrode material in various aspects. However, there are problems that it becomes difficult to steadily supply Co (cobalt) and that the material cost is higher, since Co resource is unevenly distributed on the earth and is rare.
Moreover, LiNiO2 is desirable as the positive electrode material since LiNiO2 has a high theoretical capacity and a high discharge potential and also results in the decreasing cost. However, there are problems that the discharge capacity will be reduced because the crystal structure is collapsed as a charge-and-discharge cycle is repeated, and that the heat stability is also low. Furthermore, LiMn2O4 having a normal-spinel structure is promising as the positive electrode material, because LiMn2O4 has a high potential equivalent to LiCoO2 and is able to provide a high cell capacity, and the easy synthesis of LiMn2O4 can also achieve the reduction of the cost. However, there are residual problems of the serious capacity degradation when held at high temperature for long durations, and the less stability or the insufficient cycle property that Mn may dissolve into the electrolytic solution.
Then, recently, it has been proposed that phosphate compounds of the transition metal M (M being Fe, Mn, Co, and Ni) having an olivine structure could be used as the positive electrode material (refer to Japanese Patent Laid Open No. 9-134724). Moreover, among the phosphate compounds of the transition metal M having the olivine structure, for example, LiFePO4 is also proposed for use as the positive electrode material (refer to Japanese Patent Laid Open No. 9-171827).
A volume density of LiFePO4 is as large as 3.6 g/cm3, LiFePO4 generates the high potential of 3.4 V, and the theoretical capacity thereof is also as large as 170 mAh/g. Furthermore, LiFePO4 contains one Li, which can be released electrochemically, per Fe atom in the initial state, therefore LiFePO4 is promising as the positive electrode material. However, the discharge voltage of LiFePO4 is 3.4 V, and is lower than that of the positive electrode materials used with the current secondary lithium-ion batteries, which is a problem.
Then, it has been proposed that LiMnPO4 as an olivine type phosphate compound, containing Mn which is an element with an oxidation-reduction potential higher than that of Fe as an essential component, could be used for the positive electrode material.
However, the conventional olivine type phosphate compound containing Mn as the essential component, of which the basic composition is LiMnPO4, has a problem that it is difficult to generate Mn redox. According to Journal of the Electrochemical Society, 144, 1188 (1997), it is reported that only LiMnyFe1-yPO4, in which Fe substitutes a part of Mn, is the example capable of generating Mn redox among the olivine type phosphate compounds containing Mn as the essential component.
Moreover, in the same paper, it is reported that the actual battery constituted by using LiMnyFe1-yPO4 as the positive electrode material, showed the actual capacity of about 80 mAh/g, and it cannot be said that the sufficient capacity has been obtained.
In addition, according to the above-mentioned paper, it is reported that in the actual battery constituted by using LiMnyFe1-yPO4 as the positive electrode material, the quantity of Mn, that is, “y”, over 0.5 (y>0.5) causes the capacity reduction. That is, there are problems that an increase in the Mn component of LiMnyFe1-yPO4 can obtain a high voltage, but cause a decrease in the capacity, while a decrease in the Mn component of LiMnyFe1-yPO4 in order to obtain a high capacity can not obtain the sufficient effect of a high oxidation-reduction potential. Furthermore, there is a disadvantage that the discharge voltage decrease can cause loss of interchangeability with the current secondary lithium-ion battery.
Thus, in the case of LiMnyFe1-yPO4, it is very difficult that the high capacity and the high voltage are compatible.
The present invention has been achieved in view of the above problems. It is an object of the invention to provide a positive electrode material and a battery using the same which can realize high discharge voltages and can acquire excellent charge-and-discharge properties without reducing a capacity.